Bhavik Harish Lodhia , Luk Peeters , Emanuelle Frery

The occurrence of natural hydrogen and its sources have been reviewed extensively in the literature over the last few years, with current research across both academia and industry focused on assessing the feasibility of utilising natural hydrogen as an energy resource. However, gaps remain in our understanding of the mechanisms responsible for the large-scale transport of hydrogen and migration through the deep and shallow Earth and within geological basins. Due to the unique chemical and physical properties of hydrogen, the timescales of migration within different areas of Earth vary from billions to thousands of years. Within the shallow Earth, diffusive and advective transport mechanisms are dependent on a wide range of parameters including geological structure, microbial activity, and subsurface environmental factors. Hydrogen migration through different media may occur from geological timescales to days and hours. We review the nature and timescale of hydrogen migration from the planetary to basin-scale, and within both the deep and shallow Earth. We explore the role of planetary accretion in setting the hydrogen budget of the lower mantle, discuss conceptual frameworks for primordial or deep mantle hydrogen migration to the Earth's surface and evaluate the literature on the lower mantle's potential role in setting the hydrogen budget of rocks delivered from the deep Earth. We also review the mechanisms and timescales of hydrogen within diffusive and advective, fossil versus generative and within biologically moderated systems within the shallow Earth. Finally, we summarise timescales of hydrogen migration through different regions within sedimentary basins.